Fan casings can be made of different materials, for example steel, composite. They are often made of aluminium. Repairing defects created by the abovementioned impacts has been contemplated by welding methods; but conventional welding processes, known as TIG, have been contemplated to refill these impacts but the conventional processes (TIG) do not yield satisfactory results, in particular on aluminium casings. Since such repairing is not applicable, this led, when impacts had high dimensions, to replace the full casing, or to dismount the engine.
One alternative to the full replacing of the casing is to obtain a technical validation which results in an administrative exemption, called CDR, to have the right to go on using impacted casings, possibly after slight alterations have been made on the observed defects. However, with the ageing of the fleet and the increasing number of engines in operation, these recurrent defects led to an increasing number of technical validation requests, and delays caused to restoration of parts subjected to the observed defects.
Consequently, in order to speed up the processing of this type of defects, acceptability criteria have been introduced in technical documentation. Therefore, there is now acceptance criteria of the defects discussed which allow a controller to quickly rule on the harmfulness of the defect without necessarily requiring an exemption. These acceptance criteria have been set based on statements on different casings and empirical laws allowing to rule on the specific retention of the casing as a function of its residual thickness.
These acceptance criteria have the form of trinomials of dimensions characterising the allowable defects; a first value of this trinomial is relative to a maximum allowable depth, a second value is relative to a maximum allowable length and a third value is relative to a minimum distance to be observed between two defects, in particular two defects related to an impact. Thus, for example, for an upstream zone of a casing, if a controller detects an impact having a depth 0.1524 mm (millimeters), he/she is aware that, by the existence of a trinomial associated with this depth, the defect considered should not have a length higher than 25.4 mm, and that no other defect should be present at a distance lower than 3.048 mm.
Unfortunately, these acceptance criteria only enable a restricted number defect to be accepted, because of the requirement to have available comparable reference values, and therefore do not prevent the number of requested validations from being increased; to overcome this problem, new acceptance criteria should constantly be introduced at each new noticed defect, which is not a viable solution in the long term. Furthermore, in view of the number of trinomials of values yet available, this inspection method may come out to be heavy and complex for controllers.